1. Field of the Invention
The present invention relates to a toner for developing an electrostatic image, used in an image forming process such as electrophotography, electrostatic recording or magnetic recording, and a method of fixing the toner.
2. Related Background Art
A number of methods as disclosed in U.S. Pat. No. 2,297,691, Japanese Patent Publication Nos. 42-23910 and No. 43-24748 and so forth are conventionally known as electrophotography. In general, copies are obtained by forming an electrostatic latent image on a photosensitive member by utilizing a photoconductive material, subsequently developing the latent image by the use of a toner, and transferring the toner image to a transfer medium such as paper if necessary, followed by fixing by the action of heat, pressure, heat-and-pressure, or solvent vapor. The toner that has not transferred to and has remained on the photosensitive member is cleaned by various means, and then the above process is repeated.
In recent years, such a copying apparatus is not only used as a copying machine for office work to take copies of originals as commonly done, but also has begun to be used in the field of printers serving as outputs of computers and in the field of personal copying for private use.
Under such circumstances, the apparatus are sought to be made small-sized, lightweight and highly reliable. Copying machines have now been formed of more simple components in various respects. As result, a high performance has become required for toners, and copying machines are no longer considered excellent unless an improvement in the performance of toners is achieved.
For example, various methods or devices have been developed in relation to the step of fixing a toner image to a sheet such as paper. A method most commonly available at present is the pressure-heat system using a heat roller.
The pressure-heat system using a heat roller is a method of carrying out fixing by causing a toner image surface of an image-receiving sheet to pass the surface of a heat roller whose surface is formed of a material having releasability to the toner while the former is brought into contact with the latter under application of a pressure. Since in this method the surface of the heat roller comes into contact with the toner image of the image-receiving sheet under application of pressure, a very good thermal efficiency can be achieved when the toner image is melt-adhered onto the image-receiving sheet, so that fixing can be carried out rapidly. Thus, this method is very effective in high-speed electrophotographic copying machines. In this method, however, since the surface of the heat roller comes into contact with the toner image under application a pressure, part of the toner image may sometimes adhere and transfer to the surface of the fixing roller, which is retransferred to the subsequent image-receiving sheet to cause an offset phenomenon, resulting in a contamination of the image-receiving sheet. Thus, it is considered to be one of the essential conditions in the heat roller fixing system that no toner is adhered to the surface of the heat fixing roller.
For the purpose of not causing the toner to adhere to the surface of a fixing roller, a measure has been hitherto taken such that the roller surface is formed of a material such as silicon rubber or a fluorine resin, having an excellent releasability to toner, and, in order to prevent offset and to prevent fatigue of the roller surface, its surface is further covered with a thin film formed using a fluid having a good releasability as exemplified by silicone oil. However, this method, though effective in view of the prevention of the offset of toner, requires a device for feeding an anti-offset fluid, and hence the problem that the fixing device becomes complicated.
Such complication is in the opposite direction to the demand for small size and light weight. In some instances, the silicone oil is evaporated by heat and contaminates the interior of the machine. Now, based on the idea that the fluid for preventing offset should be fed from the inside of a toner without use of any apparatus for feeding silicone oil, a method has been proposed in which a release agent such as a low-molecular polyethylene or a low-molecular polypropylene is added in the toner. Addition of such a release agent in a large quantity in order to attain a sufficient effect may cause filming onto the photosensitive member or result in a contamination of the surface of a toner carrying member such as a carrier or a sleeve, so that toner images may be deteriorated and to bring about a problem in practical use. Thus the release agent is added in the toner in such a small amount that it does not cause the deterioration of toner images. Accordingly, a releasing oil is fed in a small amount and a device by means of which the toner that may cause offset is cleaned using a member such as a web of a wind-up type are used together.
However, taking account of the recent demand for small size, light weight and high reliability, it is necessary and preferred to remove such a supplementary device. Accordingly, no countermeasure can be completely taken unless the fixing performance and anti-offset of the toner are further improved. It is difficult to achieve the improvement unless binder resins for toners are further improved.
As a technique relating to the improvement of binder resins of toners, Japanese Patent Publication No. 51-23354, for example, proposes a toner in which a cross-linked polymer is used as a binder resin. According to the method disclosed therein, the toner is effective for the improvement in anti-offset and wind-around resistance, but on the other hand an increase in the degree of cross-linking results in an increase in the fixing temperature. Hence it is difficult to obtain a toner having a sufficiently low fixing temperature, a superiority in anti-offset and wind-around resistance, and a sufficient fixing performance. In general, in order to improve the fixing performance, the binder resin must be made to have a low molecular weight to lower its softening point. This conflicts with the measure for the improvement of anti-offset. Since the binder resin is made to have a low softening point, the glass transition point of the resin is necessarily lowered with causes an undesirable phenomenon known as blocking of toner which occurs during storage.
To cope with this problem, Japanese Patent Application Laid-open No. 56-158340 proposes a toner containing a binder resin comprised of a low-molecular weight polymer and a high-molecular weight polymer. This binder resin can achieve an improvement with respect to fixing performance, but is not satisfactory with respect to anti-offset. It is difficult for this toner to completely satisfy the anti-offset and fixing performance.
Japanese Patent Application Laid-open No. 58-203453 proposes a toner containing a binder resin comprised of a low-temperature softening resin and a high-temperature softening resin. This binder resin can achieve an improvement in respect of fixing performance, but is not be satisfactory with respect to anti-offset. It is difficult for this toner to completely satisfy the anti-offset and fixing performance.
Japanese Patent Publication No. 60-20411 proposes a process for producing a resin composition comprising a polymer with a low degree of polymerization and a polymer with a high degree of polymerization. When this resin is used as a binder resin for a toner, the resin tends to achieve an improvement with respect to fixing performance, but is not satisfactory with respect to anti-offset. Thus there is much room for improvement.
In relation to a toner containing a binder resin comprised of a blend of a low-molecular weight polymer and a cross-linked polymer, Japanese Patent Application Laid-open No. 58-86558, for example, proposes a toner comprising a low-molecular weight polymer and an insoluble infusible high-molecular weight polymer as main resin components. According to the method disclosed therein, the fixing performance and grindability tend to be improved. However, it is difficult to satisfy a high performance both of anti-offset and grindability, because the weight average molecular weight/number average molecular weight (Mw/Mn) of the low-molecular weight polymer is not more than 3.5 and the content of the insoluble infusible high-molecular weight polymer is as large as from 40 to 90 wt. %. Thus, it is very difficult from a practical viewpoint to provide a toner satisfying properly fixing performance (in particular, in high-speed fixing), offset resistance and grindability unless a fixing assembly is provided with a device for feeding an anti-offset fluid. Moreover, the toner must be heat-kneaded at a temperature far higher than that in usual instances or heat-kneaded at a high shear, because the insoluble infusible high-molecular weight polymer used in a larger amount turns out to have a very high melt viscosity as a result of the heat-kneading carried out when the toner is prepared. As a result, in the instance where it is kneaded at a high temperature, the toner characteristics may be lowered because of thermal decomposition of other additives. In the instance where it is kneaded at a high shear, the molecules of the binder resin may be excessively sheared. Thus, there is the problem that the desired anti-offset can be achieved only with difficulty.
Japanese Patent Application Laid-open No. 60-166958 proposes a toner comprising a resin composition obtained by polymerizing monomers in the presence of a low-molecular poly(.alpha.-methylstyrene) having a number average molecular weight (Mn) of from 500 to 1,500.
In particular, this publication discloses that the number average molecular weight (Mn) may preferably be in the range of from 9,000 to 30,000. With an increase in Mn for the purpose of improving anti-offset, the fixing performance of the toner and the grindability at the time the toner is prepared may become more questionable from a practical viewpoint. Hence it is difficult to satisfy in a high performance both the anti-offset and the grindability properties. Thus, the toner having a poor grindability at the time the toner is prepared causes a lowering of production efficiency, and also coarse toner particles tend to be included into the toner, undesirably resulting in black spots around a toner image.
Japanese Patent Application Laid-open No. 56-16144 proposes a toner containing a binder resin component having at least one maximum value in each region of a molecular weight of from 10.sup.3 to 8.times.10.sup.4 and a molecular weight of from 10.sup.5 to 2.times.10.sup.6, in the molecular weight distribution measured by gel permeation chromatography (GPC). The toner, however, is sought to be further improved in fixing performance and anti-offset.
Japanese Patent Application Laid-open No. 63-223662 proposes a toner comprising a binder resin i) containing 10 to 60 wt. % of a THF(tetrahydrofuran)-insoluble matter, based on the binder resin, ii) having a weight average molecular weight/number average molecular weight (Mw/Mn) of .gtoreq.5, a peak in the region of a molecular weight of from 2,000 to 10,000 and a peak or shoulder in the region of a molecular weight of from 15,000 to 100,000, in the molecular weight distribution measured by GPC of a THF-soluble matter, and iii) containing a component with a molecular weight of not more than 10,000 in an amount of from 10 to 50% by weight in the binder resin. In this instance, the resin can bring about a superiority in the grindability, fixing performance, filming or melt-adhesion resistance to a photosensitive member, image forming performance, and anti-offset (in particular, anti-offset at the high-temperature side), but is desired to achieve a further improvement in its anti-offset and fixing performance for the toner. In particular, it is difficult for this resin to cope with the severe condition when both the fixing performance at low temperature and the anti-offset at a high temperature are required to be simultaneously satisfied.
In addition, it is very difficult to achieve both properties related to properties concerned with fixing and the grindability at a high level of performance. In particular, the grindability required when toners are prepared is an important factor for the recent trend in which toners are made to have a smaller particle diameter because of the demand for a higher quality level, higher resolution and higher fine-line reproduction of copied images. The improvement in grindability is also very important from the viewpoint of energy since a very large energy is required in the step of grinding.
The phenomenon that toner is melt-adhered to the inner wall of a grinding apparatus to provide poor grindability tends to occur in a toner having a good fixing performance. In the process of copying, a step of cleaning the toner remaining on a photosensitive member after transfer is required. Nowadays, from the viewpoint of making apparatus small-sized, light weight and highly reliable, it has become common to carry out cleaning by the use of a blade (i.e., blade cleaning). As photosensitive members are made to have a longer lifetime, as photosensitive member drums are made small-sized and as systems are made high-speed, melt-adhesion resistance and filming resistance to photosensitive members have been more strictly required in toners. In particular, amorphous silicon photosensitive members having been recently put into practical use are highly durable, and also organic photoconductive photosensitive members (OPCs) have longer lifetimes. Hence, the performances required in toners have become higher.
In order to make apparatus small-sized, components must be so designed as to be readily assembled. This requires that the space through which the air flows be small. In addition, heat sources for a fixing assembly and an exposure system are very close to a toner hopper or a cleaner. As a result, toner is exposed to a high-temperature atmosphere. Hence, no toner can any longer be put into practical use unless it has a better blocking resistance.
For example, the performances required in toner conflict with each other in almost all properties, as in fixing performance and blocking resistance. Yet it is increasingly sought and studied to satisfy all such properties and maintain a high performance. No satisfactory results, however, have been obtained.
As previously stated, as a method of fixing a visible image of toner to a recording medium, the heat-roll fixing system is widely used. A recording medium retaining thereon a toner visible image which has not been fixed is heated while it is held and carried between a heat roller maintained at a given temperature. A pressure roller having an elastic layer is moved into pressure contact with the heat roller.
The heat-roll fixing, however, has the following problems that should be resolved.
(1) A waiting time (a time during which an image-forming operation is prohibited) is required until the heating roller reaches the given temperature.
(2) The heating roller must be maintained at a proper temperature in order to prevent poor fixing caused by the variations of heat-roller temperatures that may occur when the recording medium is passed or because of other external factors, and also to prevent the phenomenon of offset of toner on the heating roller. This makes it necessary to make the heat capacity of the heating roller or a heater element large, which requires a large electric power.
(3) After the recording medium has been passed over the heating roller and discharged, the recording medium and the toner on the recording medium are slowly cooled because of the high temperature of the heating roller and also because the atmospheric temperature has become higher. That results in a state in which a high adhesion of the toner is maintained. Thus, there may often occur offset, or paper jam caused by the winding of the recording medium around the roller.
It is earnestly sought to develop a fixing method that has solved such problems, which requires only a short waiting time and which requires a low consumption of electric power while achieving excellent fixing of a toner latent image to a recording medium and excellent anti-offset. Such a fixing method greatly depends on toner properties, and one of the important properties is the low-temperature fixing performance of the toner.
As means for preventing occurrence of the offset phenomenon, the following methods are known as previously stated: (i) The method in which the fixing is carried out while applying a release agent such as silicone oil to the surface of a heating roller, (ii) the method in which a high-molecular weight polymer is used as a binder resin for toner, and (iii) the method in which a wax having release properties is incorporated in a toner.
In the method (i), however, when the oil is heated it generates a smell, and also a device for feeding the oil is necessary which complicates the structure.
In the method (ii), although the anti-offset of the toner can be improved, it is difficult to achieve the low-temperature fixing because of a concurrent rise of the melting temperature of the toner.
In respect of the method (iii), Japanese Patent Publications No. 52-3304, No. 52-3305, No. 57-52574, No. 53-155655 and No. 58-12580, for example, disclose toners comprising a styrene resin and a specific release agent.
Under existing circumstances, however, these toners can achieve an improvement particularly in regard to the high-temperature side offset phenomenon, but little improvement in respect of the low-temperature fixing performance. When such a release agent is used in a binder having low-temperature melting properties, it is difficult for the release agent to have the releasing effect at such low temperatures, thereby tending to cause a low-temperature offset phenomenon. This causes a reduction of fixing performance, and results in a failure to achieve low-temperature fixing performance.
To prevent the high-temperature side offset phenomenon, methods are known in which a high-molecular weight polymer is incorporated in a binder resin. The methods are disclosed, for example, in Japanese Patent Applications Laid-open No. 50-134652, No. 54-114245, No. 56-16144, No. 56-158340 and No. 58-203453.
Most of the toners used in these methods have achieved an improvement in the high-temperature side offset phenomenon, but are required to be further improved in respect of the low-temperature fixing performance. On the other hand, the low-temperature fixing performance have been achieved to a certain extent in some of these methods. A further improvement, however, is sought in respect of development performance and blocking resistance.